Activation Energy, Thermodynamics, ATP, Metabolism

Bioenergetics

The study of how energy moves and is used in living things.

How energy flows through life:

1. Plants use photosynthesis to capture sunlight and make sugar (glucose).

2. Herbivores (plant-eaters) eat plants

3. Carnivores eat herbivores, getting energy that originally came from plants.

4. Decomposers (like fungi and bacteria) break down dead plants and animals, recycling nutrients back into the environment.

Metabolism

All the chemical reactions happening inside a cell or an organism.

Metabolic Pathway

A step-by-step series of chemical reactions that turn a starting material (called a substrate) into a final product.

Metabolic Pathway Example

• Photosynthesis: CO₂ + H₂O → glucose (C₆H₁₂O₆), using sunlight.

• Cellular respiration: Breaks down glucose → releases energy → produces CO₂ + H₂O again.

• Both plants and animals use cellular respiration to get usable energy from food.

Redox Reactions

• Redox = Reduction + Oxidation
  These are reactions where electrons move from one molecule to another.

• Usually, when a molecule is reduced, it also gains a proton (H⁺) → meaning it stores more potential energy.

Reducing agent

gives away electrons.

Oxidizing agent

takes electrons.

Oxidized

loses electrons (and energy).

Reduced

gains electrons (and energy).

Memory trick (OILRIG)

OIL RIG → Oxidation Is Loss, Reduction Is Gain (of electrons).

Redox Reaction Example

Example (carbon):

• CH₄ → CO₂: carbon loses electrons → oxidized → energy lost.

• CO₂ → CH₄: carbon gains electrons → reduced → energy gained.

Evolution of Metabolic Pathways

All living things share some of the same metabolic pathways, showing a common ancestry.

• The process called glycolysis (the first step in breaking down glucose) is found in almost every organism.

• It suggests that all living things evolved from shared ancient ancestors.

Evolution of Meabolic Pathways Over Time

Over time:

• Early life used anaerobic metabolism (without oxygen).

• Later, when oxygen became available, life evolved aerobic metabolism (with oxygen), which makes much more energy.

• Enzymes became specialized to help organisms survive in their own environments.

 anaerobic metabolism

without oxygen

 aerobic metabolism

with oxygen

Activation Energy

The small amount of energy needed to start a chemical reaction.

• Think of activation energy as the push needed to get a ball rolling down a hill.

• Both exergonic and endergonic reactions need activation energy to begin

Activation Energy Example

Example (gasoline):

• Gasoline has lots of stored chemical energy.

• A spark gives it enough activation energy to start burning.

• Once it starts, it gives off enough heat to keep the reaction going.

Exergonic

Releases energy overall (spontaneous).

Endergonic

Requires energy input (non-spontaneous).

Thermodynamics

The study of how energy moves and changes in matter.

First Law of Thermodynamics

Energy cannot be created or destroyed — only changed into different forms.
The total energy in the universe stays the same.

• Plants change light energy from the sun into chemical energy (sugar).

Second Law of Thermodynamics

Every energy transfer is not 100% efficient — some energy is always lost as heat.

• This loss increases entropy (disorder or randomness).

ATP: Adenosine Triphosphate

• Cells need energy for endergonic reactions (reactions that require energy).

• ATP is the main energy currency of the cell — it powers almost everything.

• Most endergonic reactions are powered by ATP hydrolysis (breaking down ATP to release energy).

ATP Structure

adenosine + three phosphate groups.

• Adenosine = adenine (a nitrogen base) + ribose (a 5-carbon sugar).

• The three phosphates are called alpha (α), beta (β), and gamma (γ).

• The bonds between phosphates store high energy.
  When one bond breaks, energy is released because the products have lower energy than the original molecule.

ATP Hydrolysis

Reaction:
ATP + H₂O → ADP + Pi + free energy

• Pi = inorganic phosphate.

• ΔG = -7.3 kcal/mol (negative means energy is released).

• ATP is unstable and breaks down quickly.

How hydrolysis works

• energy not used → heat

• If it’s coupled with an endergonic reaction, that reaction can use the released energy.

• process is reversible — ADP + Pi can be joined again to make ATP.

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• Energy is stored in the phosphate bonds.

• Breaking a bond releases energy.

• Cells can recharge ATP from ADP again using energy from food.

Coupled

linked reactions where one gives energy and the other uses it.

Entropy

Energy transfers increase entropy → things naturally become more random over time.